Molecular Gastronomy: Exploring the Science of Flavor (20 page)

In parallel with this work, research is being conducted with a view to identi-

fying food products and processing techniques that are conducive to the devel-

opment of pathogenic strains. This means determining the levels of humidity,

temperature, acidity, and storage time associated with a risk of contamination

in order to improve industrial quality control (the legal standards for which

vary according to the product) and official supervision by government agen-

cies.

The Risks of Eating

The dissemination of food contamination information is bound to be unset-

tling for producers and consumers alike. But it is necessary because it prevents

Public Health Alerts
| 125

illness by forcing producers to take contaminated foods off the market. The

fact remains that certain foods pose a greater danger to public health than oth-

ers. Must they be banned? In Patrick Pardon’s view, there is no point trying to

prohibit the sale of products that carry a risk of
Listeria
contamination; France

will not consent to being deprived of its rillettes or raw milk cheeses. The duty

of public policy is to minimize the spread of infection by warning those who

are most vulnerable to it. Prohibition would have adverse consequences not

only for the economic stability of the food processing industry but also for the

health and well-being of the public, which would be tempted to provide itself

with banned products from unsafe sources.

126 | t he physiology of f l a vor

Investigations & Models

The Secret of Bread

Chemists look to improve bread dough by investigating the protein bonds

that form its glutenous network.

t h e b e h a v i o r o f w h e a t f l o u r can be understood by analyzing the

properties of its two main components: starch granules, which swell up in the

presence of water, and proteins, which form a glutenous network as dough

is kneaded. How do the forces among proteins contribute to the mechanical

properties of the dough? It has long been known that bonds between the sulfur

atoms found in wheat proteins play a role in structuring gluten. Other forces

have been discovered as well.

Gluten is a viscoelastic network of proteins that becomes elongated by pull-

ing and then partially reverts to its initial form when the tension is relaxed. The

quality of bread depends on the quality of its gluten. Indeed, gluten is what

makes breadmaking possible: Yeast produces carbon dioxide bubbles, with the

result that the volume of the dough increases, and the protein network of the

gluten preserves the dough’s spherical shape by retaining these gas bubbles. It

therefore becomes necessary to understand the reticulation of wheat proteins,

that is, how bonds are established between them.

As early as 1745 the Italian chemist Jacopo Becarri showed that gluten can

be extracted by kneading flour with a bit of water and then placing the lump

formed in this way under a thin stream of water. Rinsing washes away the

white starch granules, leaving the gluten between one’s fingers. It was later

demonstrated that only certain insoluble wheat proteins called prolamins

make up the glutenous network of bread.

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These prolamins are of two types: gliadins, which are composed of only

a single protein chain (a sequence of amino acids), and glutenins, which are

large structures composed of several protein subunits linked by disulfide

bridges (that is, the subunits are connected by two covalently bound sulfur

atoms). Do these disulfide bridges also link the glutenins to one another? The

traditional view is that kneading establishes supplementary disulfide bridges

between the various prolamins that break and almost immediately reform as

the baker works the dough.

The glutenins have a central domain (containing 440–680 amino acids)

formed of short repeated sequences and flanked by two terminal domains. The

size of the central domain determines the molecular mass of the glutenins;

the terminal domains contain cysteines, amino acids that bear sulfur atoms

capable of forming disulfide bridges. Nonetheless, the chemical characteristics

of glutenins do not completely explain their capacity to make gluten.

A World Made of Dough

In 1998, a team led by Jacques Guégen at the Institut National de la Recher-

che Agronomique station in Nantes showed that prolamins can bond with one

another by means of dityrosine bonds. Tyrosine is an amino acid whose lateral

chain is composed of a ch group, a benzene nucleus, and an –oh hydroxyl

2

group. Shortly afterward, on the basis of this research, Katherine Tilley and her

colleagues at the University of Kansas demonstrated the importance of dityro-

sine bonds in gluten. From bread dough at various stages of kneading, they

extracted, dissociated, and chemically analyzed the gluten of the kneaded flour

and found that concentrations of dityrosine increased during kneading. This

raised the question of what role dityrosine plays in the formation of gluten.

Further analysis disclosed the existence of two types of dityrosine bonds:

dityrosine, in which two benzene groups are linked by a neighboring carbon

atom of the –oh hydroxyl group; and isodityrosine, in which an oxygen atom

belonging to a hydroxyl group on one tyrosine binds to its carbon neighbor in

the hydroxyl group on the other tyrosine.

This discovery caused a stir among gluten chemists, for dityrosine bonds

are commonly found in plant proteins, whose sequences and structures re-

semble those of glutens, as well as in resilin proteins, found in insects and

132 | investigations a nd mod el s

arthropods, and in elastin and collagen, both found in vertebrates. In forming

dityrosine bonds by kneading dough, the baker reproduces the living world.

On the other hand, the Nantes team observed that dityrosine bonds occur

in the presence of a type of enzyme known as peroxidase, which is naturally

present in flour. Does the long working of the dough needed to make bread

cause the enzymes to react with the glutenins by giving them time to establish

dityrosine bonds? What roles do dityrosines and disulfide bridges play in the

formation of gluten?

The ability to identify these bonds raises a further question. Improved ad-

ditives can be used to facilitate kneading or intensify the production of gluten.

When oxidant compounds such as ascorbic acid and potassium bromate are

added to bread dough, for example, the number of dityrosine bonds that are

formed increases. It used to be thought that additives of this sort favor the for-

mation of disulfide bridges, but it may be that they also cause dityrosine bonds

to be created. In that case one may imagine new methods for selecting wheat

on the basis of its gluten content. Would it be enough simply to measure the

quantity of dityrosines in a certain kind of dough in order to assess the quality

of its gluten?

The Secret of Bread
| 133

36

Yeast and Bread

Bread owes its avor to fermentation.

f r e n c h b r e a d —particularly its principal representative, the baguette—is

reproached nowadays for having less flavor than it used to and for drying out

too quickly. The second criticism is unjust, for it neglects the fact that the ba-

guette was a product meant for city dwellers who could buy what they needed

several times a day at their local bakery; the crust was more important than

shelf life. Yet many bakers today admit to being more concerned with the me-

chanical behavior of the dough than the taste and odor of the bread.

The flavor of bread comes from the cooking of the dough, in which various

reactions combine to produce a crispy, flavorful crust, and from the action of

the constituent molecules of a species of yeast known as
Saccharomyces cere-

visiae
(brewer’s yeast) and the products of their fermentation in helping to

form the volatile compounds found both in dough and in bread itself. To better

understand this process, two Institut National de la Recherche Agronomique

(inra) groups in Nantes compared breads made by several different methods,

with or without yeast and with or without fermentation.

The most common method, direct yeast fermentation, begins with the

kneading of a dough composed of flour, water, yeast, and salt for about twenty

minutes. The dough is then allowed to ferment for forty-five minutes (floor-

time), at which point it is divided into lumps and fermented again for an hour

and forty minutes (proofing) before being cooked at 250°c (about 475°f) for

thirty minutes.

134 |

The sponge method (sometimes called
sur’poolish
or French starter) is iden-

tical except that the dough undergoes a process of prefermentation in a semi-

liquid environment. Water is first combined with a smaller quantity of flour

to obtain a preparation with the consistency of crêpe or pancake batter. This

preparation is left to ferment for several hours. An additional measure of flour

is then incorporated to restore a consistency equal to that of directly fermented

dough, and the same procedure followed as with the traditional method.

A third method consists of cultivating beforehand a natural microflora com-

posed of yeast and lactic bacteria. The starter thereby obtained—sourdough—is

then used to initiate the process of fermentation in bread dough.

“Vinegar” Needed

As late as the mid-1980s studies of the volatile compounds in bread re-

vealed no systematic qualitative differences from one type of bread to another,

although tasters were quite capable of distinguishing between them. Only the

proportion of certain volatile organic acids seemed to vary markedly. In particu-

lar, the sponge and sourdough methods yield acetic acid concentrations that

are, respectively, two and twenty times greater than those obtained by direct

fermentation. In the case of the sourdough method, lactic acid is also produced

by the bacteria that colonize the dough along with the yeast.

Studies of direct fermentation extended these first researches by clarify-

ing the action of yeast in bread, a complex environment that is modified by

cooking. An initial comparison of breads obtained by direct fermentation with

breads baked after inhibition of yeast, without the addition of yeast, or with

yeast added just before cooking revealed the various compounds produced by

the action of yeast in French breads. The use of yeast in fermenting dough

is directly responsible for the presence of compounds such as 3-hydroxy 2-

butanone, 3-methyl 1-butanol, and 2-phenylethanol (which has the odor of a

wilted rose).

In bread cooked without yeast, other components are more abundant than

in ordinary bread, particularly monounsaturated and polyunsaturated alde-

hydes and alcohols such as pentanol and benzyl alcohol, which may result

from the oxidation of lipids in flour (the analogue of foods that turn rancid), a

process known to contribute to the flavor of bread. The constituent compounds

of yeast appear to play only a weak role.

Yeast and Bread
| 135

Finally, the complexity of the phenomena that produce the transformation

of bread dough led the inra biopolymers laboratory in Nantes to study un-

cooked dough as well, with or without yeast, fermented or not. In this case

chemical analysis was combined with investigation of the olfactory properties

of dough extracts: Volatile compounds were extracted by various solvents, sepa-

rated by chromatography, and smelled by the experimenters through a tube.

What they discovered was a general increase in the concentration for sev-

eral alcohols, ketones, esters, and lactones and a reduction in the concentration

of aldehydes. They also found that yeasts produce higher levels of alcohol. But

chromatographic analysis, combined with sensory evaluations, showed that